This invention relates to magnetic gradient detection and in particular to an optical fibre magnetic gradient detector.
Magnetic anomaly detection (MAD) systems are used for submarine detection and geophysical survey systems. A convenient way of detecting an anomaly is to measure the gradient of the magnetic field. The Earth's field will be distorted by an anomaly so that a gradient of the ambient field results. Our co-pending G.B. applications No. 8504729 (corresponding to Extance and Jones U.S. application Ser. No. 826,883, filed Feb. 6, 1986) and No. 8504731 (corresponding to Extance and Jones U.S. application Ser. No. 827,452, filed Feb. 10, 1986) describe various gradient detector schemes whereas the present application relates to a particular form of fibre optic magnetic gradient detector.
It is well known that when certain materials are placed in a magnetic field their length changes. Such materials are said to be magnetostrictive. For small fields of magnitude H, the change in length is proportional to H.sup.2. If an optical fibre is bonded to or coated with a magnetostrictive material, or coiled around a magnetostrictive former, then the application of a magnetic field causes a change in the optical path length within the fibre, this being due to a combination of both length and refractive index changes. This path length change can be measured by incorporating the magnetically sensitised fibre in one arm of an all-fibre Mach-Zehnder interferometer and hence the magnetic field can be determined. The interferometric configuration means that small changes in fibre length may be readily detected. It is well documented that optical path length changes as small as 10.sup.-6 of a wavelength, i.e. 10.sup.-12 m may be detected. Minimum detectable fields of 1 gamma per meter of sensitised fibre have been demonstrated. Because such small length changes may be easily caused by effects other than the desired magnetostrictive one, such as temperature and pressure, the interferometer suffers from cross-sensitivity to other parameters. Since the frequency range of interest for a particular magnetic sensor is in the region 0.01 to 1 Hz, clearly the low frequency noise due to environmental fluctuations will obscure small signals. Various other parameters of the interferometric system also change on a similar time scale, such as polarisation fluctuations in the fibres and these also obscure the magnetic signal.
In conventional all-fibre Mach-Zehnder interferometers the detection of a.c. magnetic signals, where the most sensitive measurements to date have been achieved, is usually accomplished using active homodyne demodulation which maintains the interferometer at quadrature by compensating for low frequency phase variations such as those which arise from temperature. The system maintains maximum interferometric sensitivity to a.c. magnetic fields beyond the frequency range of the compensation. This signal recovery system relies on the amplitude of the magnetic signal being greater than that of the interferometer 1/f noise at the chosen signal frequency. DC magnetic field detection is a more difficult problem because the signal is in the same frequency band as the 1/f noise and one has no prior knowledge of the signal waveform. One conventional interferometer is that disclosed by Koo K. P. et al in "A Fiber Optic DC Magnetometer" IEEE J. Lightwave Technology, LT-1, 3, pp 524-5, 1983. This dc magnetometer uses an all-fibre Mach-Zehnder with a diode laser source and a passive demodulation scheme with a (3.times.3) coupler. A coil was wound around a metallic glass (magnetostrictive material) sensitised arm, and an a.c. biasing field was applied thereby. A spectrum analyser was used to measure signals at the output of the interferometer which were at the frequency of the alternating bias field. Upon the application of d.c. bias fields, simulating d.c. fields to be detected, the output was shown to be linear up to an applied field of 1 Oe and by comparing the signal due to a known field with the noise of the system the sensitivity was determined to the 10.sup.-10 tesla m.sup.-1 in a 1 Hz bandwidth. In a magnetic gradient detector both arms of a Mach-Zehnder interferometer are sensitised with magnetostrictive material and they are separated by a desired baseline spacing. Koo K. P. and Sigel G. H. describe a magnetic gradient detector in "A Fibre-Optic magnetic gradiometer" IEEE J. Lightwave Technology, LT-1, 3, p 509, 1983. Both arms of the interferometer were sensitised to the same degree and the device responded to difference in field between the two arms. The optical source and demodulation scheme of the interferometer were the same as described in the first mentioned paper, but there were two sets of excitation coils providing the alternating bias fields, both operating at the same frequency. The magnitudes and phases of the a.c. and d.c. components of these fields were carefully balanced in order to remove any common mode effect caused by variations in the ambient field level. The ability to carry out this fine tuning electrically rather than mechanically altering the coupling between the fibre and the metallic glass is particularly advantageous. The gradient sensitivity that can be achieved is similar to that of the simple total field sensor i.e. 10.sup.-10 tesla m.sup.-1 with a baseline spacing of 0.1 m.